The present invention relates to radiation therapy in which radiation is applied to a tumor or the like for treatment thereof, and in particular to adaptive/dose-guided radiation therapy where the radiation is applied and monitored according to spatially defined treatment plan and refined if the specific goals of the treatment plan are not being met.
Radiation therapy applies high-energy radiation to cancer cells to selectively destroy those cells, both by focusing the radiation on a tumor site and by relying on increased susceptibility of cancer cells to radiation. The latter, increased susceptibility, results from a diminished ability by cancer cells to repair sub-lethal DNA damage and a higher reproduction rate of cancer cells, which can cause them to be more frequently in phases of cell division that are most susceptible to disruption by radiation.
Improved outcomes in radiation therapy may be obtained by “fractionation” in which a radiation dose is applied in “fractions” spread out over multiple treatments at different times. This fractionation allows healthy cells, which are more efficient in DNA repair, to recover and increases the chance that a given cancer cell will be in a radiation-susceptible division mode at a time of treatment.
In adaptive/dose-guided radiation therapy, a spatially defined treatment plan is prepared defining a desired dose. The treatment plan is defined on an image of the patient, for example taken by CT, PET or MRI imaging or the like, and used to align and control the radiation beams with respect to the patient during each fractional treatment. During treatment, the amount of dose received by each region of tissue during each treatment fraction is monitored. This monitor dose for each treatment fraction may be combined to track the progress of the treatment in meeting the goals of the treatment plan.
The registration of the treatment plan with the patient and the monitoring of radiation dose over time are complicated by changes in the patient and tumor over time, for example, caused by weight loss and tumor shrinkage. For this reason, it is known to acquire new images of the patient at the time of each treatment fraction and to adjust the radiation plan to fit the current image so that radiation is applied correctly to the tissue despite changes in location and area. This yields the measured dose fraction. It is desirable to obtain the combined delivered dose from all treatment fractions to date. Measured dose fractions may be warped to a common reference frame based on one image before being combined. Generally, the treatment plan and measured dose fractions will be termed “image-type” data, reflecting the fact that they share a common coordinate structure with image data and are normally registered with an image.
Registering image-type data by warping or deforming the image-type data may be provided by comparing images taken at different times during the treatment and developing a spatial mapping of corresponding volume elements between images to provide a deformation map. The deformation map may be a deformation vector field (DVF) or a set of points from which mathematical deformations can be derived (e.g. thin plate splines) or other known techniques which describes how the tissue has deformed between images. Such deformation maps can be obtained by wide variety of techniques including for example correlating subregions of the image in a block matching process that determines vectors for the deformation of each block according to regions of highest correlation.
Minor errors in measuring and calculating the deformation maps can make the results of image transformation using deformation maps highly dependent on the sequence of the warping operation. For this reason, for example, warping image A to image B and then warping image B to image A will not yield the original image A. This error is termed “inverse inconsistency error”. Further warping image A to image B to image C and then back to image A will not yield the original image A nor will warping image A to image C yield the same image as warping image A to image B and thence to image C. These errors are termed “transitivity errors”.
These errors can create uncertainty and confusion in the treatment planning process. One approach to correcting this problem, described in Skrinjar, O., Bistoquet, A., and Tagare, H., Symmetric and Transitive Registration of Image Sequences, Int J Biomed Imaging, 2008, article ID 686875, enforces the use of a predetermined reference image through which all transformations must be undertaken.